Metal strain gages are used for sensing strain at a surface to which the strain gage is mounted. The amount of strain is determined on basis of the change in the electrical resistance of an electrical circuit in the strain gage. Typically, the circuit is formed by a thin metal foil or a thin metal conductor arranged in a serpentine pattern. As the surface under investigation is strained, deformation in the strain gage causes a change in the electrical resistance of the circuit. Strain gages are said to have a strain factor, resistance-strain sensitivity or a gage factor (GF).
The GF is the ratio of fractional change in electrical resistance to the mechanical strain. The GF can be represented by:GF=(ΔR/R)/εwhere ΔR is the change in resistance caused by the strain sensed by the strain gage, R is the resistance of the unstrained strain gage, and E is the strain. Additional details about the GF are available in ASTM E251.
For many metal resistance strain gages, the GF is no more than 4, and in many cases is 3 or less. In some applications, it is desirable to have a metal resistance strain gage with a GF greater than 5. Some of the benefits of a strain gage with a high GF are an enhanced signal to noise ratio and the ability to measure low levels of strain.
Among the known metal resistance strain gages, gages made from copper, nickel and manganese (Cu-44Ni-2Mn) have a GF of approximately 2; gages made from iron, nickel, and chromium (Fe-36Ni-7Cr) have a GF of approximately 3; and, gages made from platinum and tungsten (Pt-8W) have a GF of approximately 4. Metal resistance strain gages with a GF greater than about 4 have not been available; however, there are non-metal resistance strain gages available with a GF greater than 4, but they have disadvantages such as high resistance-temperature sensitivity (thermal output) and non-uniform properties from gage-to-gage. Furthermore these non-metal resistance strain gages are brittle and must be handled carefully to avoid breakage.
Accordingly, a need exists for a metal resistance strain gage with a GF of 5 or greater.